Vapor target for particle accelerators



y 0, 1968 R. 5. BROWN, JR.. ETAL 3,395,302

VAPOR TARGET FOR PARTICLE ACCELERATORS 2 Sheets-Sheet 1 Filed Jan. 10, 1966 INVENTORS MARTIN ROOS RICHARD S. B5OWN2 BY f %ATTORNEY y 30, 8 R. 5. BROWN, JR. ETAL 3,395,302

VAPOR TARGET FOR PARTICLE ACCELERATORS 2 Sheets-Sheet 2 Filed Jan. 10, 1966 mm v 9. i a

INVENTORS MARTIN ROOS RICHARD 3 BY W ToRNEY United States Patent 3,395,302 VAPQR TARGET FOR PARTICLE ACCELERATORS Richard S. Brown, Jr., (Iarnbridge, and Martin Roos,

Woburn, Mass., assignors to High Voltage Engineering Corporation, Burlington, Mass, 11 corporation of Massachusetts Filed Jan. 10, 1966, Ser. No. 519,759 8 Claims. (Cl. 31334) ABSTRACT OF THE DISCLOSURE Apparatus for altering the charge of particles, such as ions, atoms, or molocules, in a moving beam which comprises a target canal in which vapors may be inserted to intercept the beam and alter the charge of the particles comprising the beam and providing, at the ends of the canal, means for condensing and solidifying the vapors escaping from the canal.

This invention relates generally to the bombardment of a target by particles artificially accelerated and more particularly to a vapor target adapted to be bombarded by such particles.

In order to introduce charge changing in particle accelerators, fast particle beams have, in the past, been passed through thin foils or gas filled chambers. Such charge changes are quite satisfactory under certain conditions and unsatisfactory under other conditions. For example, foils are unsuitable at high currents because their lifetime is limited to a few hundred micro-amperehours. Gas fed chambers, on the other hand, are often made in the form of a low conductance canal and introduce a stream of gas into the vacuum system. If the gas canal is situated in a region where pumping is poor, the flow of gas from the canal must be minimized. This is usually accomplished by forming the canal as a tube of small diameter in long length. The transmission of particle beams through small diameter canal is a major technical problem.

The present invention overcomes all the difiiculties attendant with both the thin foils and gas canals and provides not only a longer lifetime but also better vacuum conditions and a larger diameter canal to reduce the technical problems of particle beam transmission.

Broadly speaking, these features are achieved in the present invention by providing a target canal in which vapors may be inserted and providing at the ends of the canal means for condensing and solidfying the vapors escaping from the canal.

The invention further provides a means whereby the effective target thickness may be varied and whereby the diameter of the target may be significantly enlarged over that of the prior art.

The invention still further provides a means whereby the vacuum conditions existing around the canal may be significantly improved.

These and other features and advantages of the present invention may best be understood from the following detailed description taken in conjunction with the accompanying drawings, wherein:

FIGURE 1 is a schematic drawing of a high voltage accelerator of the type that can utilize the invention;

FIGURE 2 is a schematic view of one embodiment of the present invention;

FIGURE 3 is an enlarged view of an actual beam canal and associated condensers made in accordance with the present invention taken along the beam axis;

FIGURE 4 is a sectional view of the canal shown in FIGURE 3 taken along the lines 4-4; and

3,395,302 Patented July 30, 1968 ice FIGURE 5 is a sectional view of another embodiment of the invention.

I Before beginning the detailed description of the invention, it should be clearly pointed out that a vapor is distinguished from a gas and each term is used in the description in its exact technical sense.

Thus, the term vapor is used herein, to define a substance, which, although having the form of a gas, is below its critical temperature. The term gas is used to refer to a substance which is in the gaseous state and is above its critical temperature.

With these facts in mind, reference is now made to the drawings and more particularly to FIGURE 1, wherein there is shown an idealized, high voltage accelerator of a type which will utilize the invention. This accelerator basically comprises an ion source 10, a pressure vessel 11 containing a column 12, a pair of analyzing magnets 13 and 14, a switching magnet 15, a neutralizing charge exchanger 16 and a target 9. The column 12, itself, comprises a pair of accelerator tubes 17 and 18 joined by a high voltage terminal 19 in which an electron adding charge exchanger 20 is enclosed.

The detailed operating principles of such accelerators have previously been described in the literature. It is therefore sufficient for the present to outline the basic mode of operation of such a machine. The positive ion source 10, at ground potential, generates a positive ion beam 21 which is accelerated to a low velocity by an extraction electrode (not shown). The beam is then deflected by the analyzing magnet 13, along the axis of the acceleration tubes 17 and 18 and through the neutralizing charge exchanger 16 in which positive ions in beam 21 receive an electron to produce a beam 22 of neutral particles. These particles are then allowed to coast, unaffected by the electric field, down tube 17 to the negative high voltage terminal 19. Inside terminal 19, the neutral particle beam 22 passes through the electron adding charge exchanger 20 where the neutral particles capture an additional electron and become the negative ions which form the negative ion beam 23. This negative ion beam 23 is then accelerated down tube 18, by the electric field existing between terminal 19 and ground, through the second analyzing magnet 14 and the switching magnet 15 to target 9 where they are used for well-known purposes. The described system has one dominant advantage, in that the ion source 10 is conveniently located externally to the tank and both it and the tank are at ground potential. Thus, the ion source has maximum asseccibility and versatility.

Basically, such accelerators utilize the knowledge of charge changing characteristics of ions. That is, they use the fact that electrons can be added to or stripped from particles. In the past. such adding or stripping had been accomplished by passing the beam to be changed through either a thin foil or a gas or both.

Because of the well-known, previously described, difficulties encountered with such gases and foils, the present invention was conceived, built and operated.

It should be thoroughly understood that, although the following description concerns itself with only the electron adding charge exchanger 20, its features and advantages are also applicable to the neutralizing charge exchanger 16. The invention furthermore has been successfully used for both adding electrons to negative ions and neutrals and for stripping electrons from negative ions. Thus, the present invention is quite versatile for all conditions necessary in such charge exchangers.

With these facts in mind, attention should now be turned to FIGURES 2, 3 and 4 which show one form of the electron adding charge exchanger 20 which is located in the high voltage terminal 19 between the acceleration tubes 17 and 18. This adding exchanger 20 comprises a vacuum tight housing which is connected to the acceleration tubes 17 and 18. Inside the housing is a charge exchange canal 26 which has located at either end condenser chambers 27 and 28. Passing through the housing 25 into the canal 26 is a vapor feed line 29 which is surrounded by a temperature differential block 31. Also passing through the housing 25 is a tube 32 which is connected to the condensers 27 and 28. Refrigerant, from a suitable source (not shown), is passed through this tube to cool the condensers to the desired temperature. Located at each end of the housing 25, between the housing and the acceleration tubes, are valves 33 and 34.

The device operates as follows. With valves 33 and 34 opened, the acceleration tubes 17 and 18, housing 25, canal 26, and condensers 27 and 28 are evacuated by suitable means, not shown, to a pressure of about 10 torr. The condensers 27 and 28 are then cooled by any number of satisfactory means. One preferred method consists of introducing a refrigerant, such as liquid nitrogen, into tube 32 surrounding the condensing chambers.

Simultaneously, a vapor, such as water vapor, is permitted to flow into the canal 26 through the vapor feed line 29 from a suitable source, not shown. The vapor so injected immediately expands in canal 26 and, because of the vacuum and the cold surfaces of the condensers at either end of the canal, flows in both directions towards the condensers 27 and 28. When the vapor enters the cooled condensers, substantially all of it is immediately condensed and frozen so that it does not enter the acceleration tubes but instead falls as a solid condensate to the bottom of the condenser. The amount of vapor not frozen that enters the acceleration tubes is negligible and does not deleteriously affect the pressure in the tubes. Meanwhile, the ion source 10 has been activated so that a neutral beam is now being fed down the acceleration tube 17-and through the vapor in the canal. The passage of the particle beam through the vapor causes electrons to be added to a certain percentage of the particles comprising the beam to convert them to negative ions. The negative ions are then accelerated down tube 18 to the target position where they are used. The described invention not only prevents undesirable flow of target material down the tube, but also improves the charge exchanging efliciency of the canal. The invention further provides a means by which a variable thickness of target vapor in the canal may be achieved. Still further, the invention provides a wider range of pressures than those obtainable in the prior art gas channels, for in the present invention the water in the boiler can be heated as well as metered.

The purpose of block 31 is to prevent the central portion of canal 26 from being cooled to the point where the vapor feed line 29 becomes blocked by frozen condensate. To fully assure that the vapor does not freeze in the line 29, the block 31 is opened at the top to the high pressure gas surrounding the column 12 in tank 11. To prevent leakage of this gas into the housing 25 and canal 26, it is necessary that the block 31 form a vacuum tight seal with the housing 25 and with the canal 26.

It is also desirable that the condensers do not make intimate contact with the walls of the canal. To this end, the condensers are insulated from the canal by suitable means. One highly practical means of thermally isolating the canal from the condensers is by providing a small space 24 between the canal and condensers.

It has further been found desirable that the ends of the canal be cut at an angle of approximately 45 to the axis of the canal. Thus, the bottom of the canal 26 is shorter than the top of the canal. This arrangement causes the major portion vapor to be directed downward toward the bottom of the condenser as it leaves the canal.

Furthermore, because the condensers are cooled to such low temperatures, they act as condensation or cryo-pumps which also enhance the trapping of various vapors within the vacuum system outside the condensers by solidifica- 4 tion of the vapors on the outside of the condensers. Thus the vacuum in the acceleration tubes is not degraded in any manner whatsoever but rather enhanced. This arrangement further permits the diameter of the canal 26 to be significantly enlarged such that the technical problems of beam confinement are greatly reduced.

When the condensed vapor builds up within the condensers, it becomes necessary to remove the condensed vapor from the condensers so that they will operate efficiently. To this end, it is desirable that there be in serted between the housing 25 and the acceleration tubes 17 and 18 a pair of valves 33 and 34 which can effectively seal off the housing 25 from the acceleration tubes 17 and 18. Thus, when it is desired to remove the frozen condensate from the condensers, the beam is stopped by shutting off ion source 10 and housing 25 is sealed off from the acceleration tubes by means of valves 33 and 34. When the valves 33 and 34 are actuated. and housing 25 is sealed off, the refrigerant is stopped from flowing through and into tube 32. Heating means, not shown, are then applied to the condensers 27 and 28 to liquefy the condensate contained therein. Once the condensate is liquefied, it is permitted to flow out of the condensers through drains 37 and 38 into housing 25. This fluid is then extracted from housing 25 through drain 39. If desired a pump could be attached to drain 39. Once the liquid has been completely drained out of the condensers and the housing, the device is reactived. Reactivation begins by introducing the refrigerant around the condensers to cool them down and to freeze any small residue of liquid or vapor which may be remaining in the condensers or in the housing. Once the condensers are cooled to the desired temperature, and the remaining vapor condensed, the valves 33 and 34 are opened and the device exposed to the vacuum present in the acceleration tubes. However, because the condensers have been cooled sufficiently before the valves are opened, there is no possibility of there remaining within the housing 25 any liquid or vapor. Following the opening of the valves, vapor is again allowed to fiow into the canal and the ion source reactivated to transmit down the acceleration tubes a particle beam. Thus, it is absolutely necessary that upon reactivation of the device that the condensers be cooled to their operating lever prior to the opening of housing 25 to the acceleration tubes and prior to the introduction of vapor into canal 26.

It should be thoroughly understood that although water vapor has been described in conjunction with the preferred embodiment that many other materials such as ammonia, alcohol, carbon dioxide and certain hydrocarbons can be successfully used. It is, however, necessary that the material to be used must when in its solid state, in the condensers, have a vapor pressure which is at least one order of magnitude lower than the vapor pressure existing in the evacuated acceleration tubes 17 and 18.

Although liquid nitrogen has been described as the cooling medium, it shoud be understood that any other suitable refrigerant could be used and, if desired, a standard refrigerator could be provided with its cooling coils disposed around each of the condensers provided in the invention. In such an event, the refrigerator so used must be sufficient to freeze the vapor being utilized and further must be sumcient to cool the frozen condensate such that its vapor pressure is less than the vacuum pressure existing in the accelerator tubes 17 and 18.

Although liquid helium will provide a colder temperature than liquid nitrogen, it is expensive and for this reason would not normally be used in the present invention. However, under certain conditions and for certain uses with certain vaporizable material, its use would be warranted.

With reference now to FIGURE 5, a still further embodiment of the present invention will be described wherein there is shown a water vapor jet target for the charge changing of fast ion beams. In this schematic view, boiler 40 holding a suitable supply of water 41 is heated by an immersion heater 42. The water temperature is monitored by thermocouple 43 so as to establish with accuracy the desired water temperature and related vapor pressure. When the water is heated, the water vapor expands in the needle valve orifice 44 and reaches a sonic velocity. Because this rapid expansion and increase in velocity adds kinetic energy to the water vapor, there is accompanied reduction in vapor temperature and pressure. For this reason, the needle valve orifice must be kept in good thermo contact with the hot boiler in order to prevent ice formation at the orifice. Beyond the needle valve, the kinetic energy the vapor is dissipated by turbulence in the expansion chamber 45, the Walls of which are at boiler temperature. Because at turbulence in the chamber friction causes the vapor temperature to rapidly return to very nearly its original temperature but at a greatly reduced pressure. Thus, a superheated vapor is formed in the expansion chamber 45. This vapor is then accelerated to sonic velocities by the nozzle 46. The velocity to which the vapor is accelerated depends upon the nozzle shape. Meanwhile, the condenser 47 is being cooled by liquid nitrogen 48 which is under a tank pressure of 200 p.s.i. At this pressure, the boiling temperature of the liquid nitrogen is minus 164 C. Thus, the condenser with its large outside surface acts as a sorption pump with a pumping speed of approximately 30,000 liters of water vapor per second. The vapor rapidly passes through orifice 46 and froms a jet of vapor shown as 49 within the condenser 47. Simultaneously, a beam shown as arrow 51 is passing from right to left through the vapor jet 49 wherein the particle beam undergoes charge changing. The vapor rapidly condenses in the bottom of the condenser and builds up as ice 52.

Other modifications and variations are possible, for example, the condensers 27 and 28 could be made double walled with a direct nitrogen feed thus eliminating tube 32 and instead of drawing the liquid from the heated condensers they could be pumped out.

Having thus described several preferred embodiments of the present invention, it is desired that the present invention be limited only by the following claims.

What is claimed is:

1. Apparatus for altering the charge of particles in a moving particle beam comprising, beam canal means, means for passing the beam through the canal, means for introducing a vapor into the canal to intercept the beam passing therethrough to alter the charge of said particles, and condenser means for forming a solid condensate of said vapor after said vapor has intercepted said beam.

2. The apparatus of claim 1 wherein said canal comprises a hollow cylinder open at each end and having its axis parallel to the beam, said introducing means is located between the ends of said canal, and said condensing means comprises a pair of drums larger in diameter than the said canal, one of said drums surrounding one end of said canal and the other drum surrounding the other end of said canal, each of said drums having refrigerating means coupled hereto whereby the vapor flowing from the ends of said canal is solidified.

3. The apparatus of claim 2 wherein each of said drums has refrigerating means coupled thereto and aligned apertures in each end such that the beam may pass unimpeded through said drums.

4. The apparatus of claim 2 wherein said drums are double walled having a cavity between said Walls, said cavity being filled with said refrigerating means.

5. The apparatus of claim 1 wherein said vapor is introduced into said canal at a speed lower than 738 miles per hour.

6. The apparatus of claim 1 wherein said beam canal has an inner wall and outer wall, said walls being in spaced relationship and providing a cavity therebetween, said cavity being filled with a refrigerant.

7. The apparatus of claim 2 wherein said cylinder ends are disposed at an angle of 45, to the cylinder axis, and disposed within the condensing means.

8. The apparatus of claim 3 wherein said vapor is water vapor.

References Cited UNITED STATES PATENTS 2,816,243 12/1957 Herb et al. 31363 2,885,584 5/1959 Van de Graaif 313-231 X 3,136,908 6/1964 Weinman 313-63 3,243,640 3/1966 Eichenbaum 313-231 X JAMES W. LAWRENCE, Primary Examiner. R. L. JUDD, Assistant Examiner. 

